The traditional techniques of optical time domain reflectometry (OTDR) involve the use of a single optical pulse, produced by a laser source, which is coupled to a DUT, for example an optical fiber, or another dielectric waveguide, within which the pulse propagates; the energy of this pulse is partially backscattered during its path owing to physical phenomena that occur in the structure of the DUT, in particular owing to the inelastic scattering known as “Raman scattering”.
The inelastic backscattering processes owing to Raman interaction generate two spectrally-separate components of the optical response, respectively called “Anti-Stokes Raman line” (AS) and “Stokes Raman line” (S), which are shown in FIG. 1 with reference to the wavelength λT of the laser source.
It is known that the intensity of the backscattered Anti-Stokes line (at the wavelength λAS) depends on the temperature, which can thus be measured from this intensity. It is likewise known that in order to effectively distinguish the variations of temperature from the variations of loss along the optical fiber probe, usually what is monitored is the ratio of the intensity of the Anti-Stokes (λAS) and Stokes (λS) Raman lines, or, alternatively, the ratio of the intensity of the Anti-Stokes line (λAS) and the backscattered Rayleigh line at the same wavelength λT of the laser source.
The backscattered optical power is measured by the apparatus and is then related to the time elapsed since the moment of coupling to fiber of the optical pulse, in order to obtain its spatial distribution.
The duration of the measurement of backscattering (this measurement is also known as “OTDR trace”) depends directly on the length of the DUT and is called “time of flight”. The total measurement time, on the other hand, which is necessary for the apparatus to reconstruct the spatial distribution of the physical parameter along the DUT, is represented by an integer multiple of the above mentioned time of flight, since, in general, measurement apparatuses collect multiple acquisitions of OTDR traces in order to calculate an average of them.
Such techniques are widely applied in telecommunications and also in the field of civil and industrial engineering, in apparatuses adapted to monitor very large structures, such as motorway and rail tunnels, oil and gas pipelines, power lines and large-scale industrial plants in general. In particular, such apparatuses usually comprise an optoelectronic measurement device provided with an optical fiber probe (DUT or Device Under Test) of considerable extension, usually of the order of a few tens of kilometers. In use, such optical fiber is stably coupled to and kept substantially in contact with portions or components of the engineering structure of which it is intended to monitor the respective physical parameters, such as the temperature.
The performance levels that can be obtained with traditional OTDR techniques are mainly limited by the energy of the optical pulse. Such energy is limited both by the maximum peak power of the optical pulse that can be generated with commercial laser sources and which can be used without incurring unwanted non-linear effects, and also by its duration, which cannot be increased without consequently worsening the spatial resolution of the measurement, i.e. the minimum portion in length of DUT on which it is possible to measure the physical parameter in question. So there is a typical trade-off between spatial resolution (i.e. the minimum spatial extension of a peak that can be measured with accuracy) and the signal-to-noise ratio (SNR) (i.e. the precision with which the measurement is made).
In order to overcome such limitations, it has been proposed to couple not a single optical pulse to the DUT but a suitable binary sequence of optical pulses, which is selected from families of pseudorandom codes or of complementary correlation codes. In this case, the trend over time of the backscattered optical power measured by the apparatus is given by the linear sum of the optical backscatter responses of each individual pulse that constitutes the code word. The optical backscatter response of the single pulse, which is generally used in order to obtain the spatial distribution of the physical value of the DUT, is then found using a decoding operation which depends on the type of code that was selected.
The coding gain (CG), which is defined as the ratio, for the same measurement time, of the SNR obtained with the encoding technique to the SNR obtained with the single pulse technique, has a trend as the length L of the code word varies, which shows that the improvements in SNR of the measurement apparatus decrease as L increases. Increasing the length of the code word L has the disadvantage of also increasing the duration of the time of flight, and consequently the total measurement time, by a time tf, which is directly proportional to L for a factor equal to the duration of the single optical pulse.
A further disadvantage of the above mentioned encoding techniques lies furthermore in the fact that the decoding operations increase in complexity quadratically with respect to the length L, and this makes the decoding operation unperformable by commercial components.
Therefore, the foregoing considerations lead to the conclusion that, in practice, the above-mentioned encoding techniques are effective for word lengths L that are less than a limited value, and that commercial measurement apparatuses cannot use a length L as big as desired in order to increase their measurement distance.
The aim of the present disclosure is to provide a method and an apparatus that are capable of improving the known art in one or more of the above-mentioned aspects.